1. Field of the Invention
The present invention relates to a charged particle beam apparatus.
2. Description of the Related Art
In a drawing method which uses a plurality of electron beams, it is necessary to periodically measure and correct the characteristics of the electron beams in order to reduce the influence of variations and temporal changes in characteristics of the electron beams. If each electron beam has a size sufficiently larger than that of each pixel of a commonly available image sensor, the characteristics of the electron beam can be directly measured using the image sensor. However, in practice, the size of each electron beam is as small as several tens of nanometers, so the characteristics of the electron beam cannot be directly measured.
To measure the characteristics of electron beams, a measurement scheme which uses, for example, knife edges is effective. However, when a plate including knife edges is directly irradiated with electron beams, the positions of the knife edges change with a change in temperature of the plate. The change in position of the knife edges due to heat leads to a measurement error of the irradiated positions of the electron beams. To correct this error, there may be a method of measuring the positions of the knife edges with electron beams. Nevertheless, it may also pose the problem that the irradiated positions of electron beams for measurement change, thus generating a measurement error of the positions of the knife edges. Since the position accuracy of the knife edges and the irradiated position accuracy of the electron beams influence each other, the positions of the knife edges and the irradiated positions of the electron beams must be measured independently of each other.
As a method of monitoring a change in characteristics of the knife edges, a method of comparing measurement values with normal measurement values has been proposed in Japanese Patent Laid-Open No. 2006-194618. Japanese Patent Laid-Open No. 8-37141 proposes a method of monitoring the intensity of a beam eclipsed by a plurality of apertures in an optical system to specify the position to which the beam has drifted. However, neither Japanese Patent Laid-Open No. 2006-194618 nor 8-37141 describes how to accurately measure a fluctuation in position of the knife edges using electron beams having irradiated positions which may fluctuate.